The emissions benefit in catalytically treating internal combustion engine exhaust gas, such as by passing the exhaust gas through a catalytic converter is generally accepted. The catalytic treatment process involves a chemical reaction including the storage and release of oxygen derived from the addition of known base metals to a wash coat of the converter substrate. As a catalytic converter ages, its capacity to react with engine emissions in the storage and release process may deteriorate, reducing conversion performance. Such performance may degrade to the extent that the converter no longer provides adequate reduction in engine emissions.
It has been proposed that the oxygen storage and release capacity of conventional catalytic converters may be monitored to diagnose the performance of the converter. Such monitoring has included placement of a conventional oxygen sensing element in the engine exhaust gas path in position to sense the oxygen content in catalytically treated exhaust gas. This post-converter oxygen sensor has been applied in a diagnostic described in Society of Automotive Engineers SAE paper Nos. 900062 and 910561, wherein the amplitude content of the post-converter oxygen sensor has been analyzed via an "area ratio method." This method compares the area under an envelope defined by the post-converter oxygen sensor output amplitude over a predetermined period of time to a similar area defined for a pre-converter oxygen sensor.
Such diagnostic does not account for the inherent significant variation in the area under such envelopes. As such, the diagnostic is prone to misdiagnosis of converter performance, which may result in an inconvenient and costly catalytic converter replacement, or extension of a period of increased vehicle emissions. Accordingly, what is needed is a catalytic converter diagnostic that accurately characterizes catalytic converter performance.